(a) Field of the Invention
The present invention relates to a powder type electroluminescent (EL) device and, more specifically, to a powder type EL device having a drive circuit in which charging and discharging of a capacitive EL element is effected by a high DC voltage obtained by step-up of a DC supply voltage.
(b) Description of the Related Art
A powder type EL element, a capacitive load in a sense for a drive circuit, has a structure of a capacitor in which a layer of a luminous body is sandwiched between two electrodes. The luminous body or layer is made of an dielectric material including a fluorescent material dispersed therein. When a surge voltage changing in magnitude thereof with time is applied between the electrodes of the EL element, the EL element is rendered luminous by a surge electric field generated by the surge voltage, energizing the fluorescent material in the luminous layer. The powder type EL element is generally driven by a drive circuit called an inverter.
FIG. 1 shows an equivalent circuit of a conventional powder type EL device. The drive circuit of the EL device generally designated at 10 comprises a step-up circuit 12 having oscillator 14 of a relatively short repetitive period and receiving a DC supply voltage from battery 16 to provide a surge-pulse train to an EL element 18, discharge section 20 including discharge transistor 22 for discharging electric charge from the EL element 18 to the earth potential, and a control section 24 including a second oscillator for controlling the step-up circuit 12 and discharge transistor 22 periodically. The step-up circuit 12 generates a periodical surge-pulse train including a series of surge pulses having a uniform energy among the individual pulses of the pulse train. The control section 24 controls the step-up circuit 12 and discharge transistor 22 to operate alternately, thereby effecting charging and discharging of the powder type EL element 18 alternately.
During a charge period of the EL element 18, the EL element is gradually charged up by a series of surge pulses. After the charge period continued in a certain amount of time to inject electric charge to the EL element 18, a discharge period starts so that electric charge accumulated by he pulse train on the EL element is discharged to the ground during the discharge period through discharge transistor 22.
The amount of the electric charge injected by one surge pulse depends on the magnitude of the voltage of the EL element appearing between the electrodes thereof. That is, the lower the voltage of the EL element is, the larger the amount of electric charge is injected by a surge pulse. An equivalent resistance for electric energy lost or consumed for the electroluminescence in the EL element can be regarded as a resistor connected between he electrodes in parallel to the capacitive EL element, so that the energy loss or consumption in the EL element increases in proportion to the square of the voltage appearing between the electrodes of the EL element. The voltage of the EL element during the charge period increases along a logarithmic saturation curve, resulting in that the voltage increment per unit time decreases with time to substantially zero eventually.
Assuming that the equivalent resistance for the energy loss of the EL element is constant, the larger the equivalent capacitance of an EL element is or the smaller the energy of the surge pulses is, the longer is required for a time period for voltage rise during which the voltage of the EL element rises from the ground level up to its saturation level.
In case where the charge period determined by the frequency of the second oscillator in the control section is set longer than the time period for voltage rise of the EL element, a saturation period during which the voltage of EL element remains in the saturation level continues for a long time. During such a saturation period, the EL element is scarcely luminous since the EL device can be luminous substantially only when he voltage of the EL element rises or falls rapidly. Hence, in this case, the luminous intensity of the EL element is very low because of the short luminescent period per one operation cycle including a charging and a discharging periods.
Moreover, since the EL element has a tendency that the equivalent capacity thereof reduces with age, the saturation period, which does not function for luminescence, per one operation cycle becomes large with age, resulting in a deterioration in he luminous function of the EL element additionally to the low luminous intensity as described above. Such a deterioration reduces the life of an EL device.
On the other hand, in case where the charge period determined by the frequency of the second oscillator is set smaller than the time period for voltage rise of the EL element, the charge period comes to an end before the voltage of the EL element rises to a value sufficient for luminescence, resulting in an insufficient luminescence in the EL element because of the low voltage of the EL element.
Another problem in the conventional drive circuit as described above is that, since the charge period is constant while the energy of a surge pulse depends on a supply voltage, the voltage of the EL element increases with the increase of a voltage supplied by a battery, resulting in a possibility of an ultimate break down in the EL element.
FIG. 2 shows another type of a conventional drive circuit for a powder type EL element. The drive circuit generally designated at 30 comprises a step-up circuit 12 such as shown in FIG. 1, a switching circuit 32 having two serially connected pairs of transistor 34, 38 and diode 36, 40, a control section 42 for controlling the step-up circuit 12 and the switching circuit 32, a serial resonance circuit including a reactance of reactor 42 and a capacitance of the powder type EL element 18 to be driven and having one of the terminals connected between the pairs of transistor 34, 38 and diode 36, 40, and a pair of serial capacitors 44 and 46 for maintaining the other of the terminals of the serial resonance circuit at an intermediate potential of the output of the step-up circuit 12.
In operation, the high DC voltage generated by the step-up circuit 12 is fed to one of the electrodes of the EL element 18 periodically through the switching circuit 30 and the reactor 42. Then, the energy of the resonance circuit is discharged to the earth potential complementarily with the charging. The frequency obtained by the operation of the switching circuit 30 is relatively low, i.e. in a range between 500 and 1000 Hz.
In the second conventional drive circuit 30 for an EL element 18, since the operating frequency of the switching circuit 30 is low, the reactor 42 provided for serial resonance circuit should have a large reactance for obtaining serial resonance, so that the size of the reactor 42 becomes large resulting in a large area for the drive circuit.